Method for distributed power harvesting using dc power sources

ABSTRACT

A system and method for combining power from DC power sources. Each power source is coupled to a converter. Each converter converts input power to output power by monitoring and maintaining the input power at a maximum power point. Substantially all input power is converted to the output power, and the controlling is performed by allowing output voltage of the converter to vary. The converters are coupled in series. An inverter is connected in parallel with the series connection of the converters and inverts a DC input to the inverter from the converters into an AC output. The inverter maintains the voltage at the inverter input at a desirable voltage by varying the amount of the series current drawn from the converters. The series current and the output power of the converters, determine the output voltage at each converter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/950,307, filed Dec. 4, 2007, and claims priority to U.S. ProvisionalPatent Applications, Ser. No. 60/868,851, filed Dec. 6, 2006, and titled“Distributed Solar Array Monitoring, Management and Maintenance,” Ser.No. 60/868,893, filed Dec. 6, 2006, and titled “Distributed PowerHarvesting System for Distributed Power Sources,” 60/868,962, filed Dec.7, 2006, and titled “System, Method and Apparatus for ChemicallyIndependent Battery,” Ser. No. 60/908,095, filed Mar. 26, 2007, andtitled “System and Method for Power Harvesting from Distributed PowerSources,” and Ser. No. 60/916,815, filed May 9, 2007, and titled“Harvesting Power From Direct Current Power Sources,” the entire contentof which is incorporated herein by reference. Further, this applicationis related to ordinary U.S. patent application Ser. No. 11/950,224titled “Current Bypass for Distributed Power Harvesting Systems Using DCPower Sources,” patent application Ser. No. 11/950,271 titled“Distributed Power Harvesting Systems Using DC Power Sources,” patentapplication Ser. No. 11/951,419 titled “Monitoring of Distributed PowerHarvesting Systems Using DC Power Sources,” patent application Ser. No.11/951,485 titled “Removable Component Cartridge for IncreasingReliability in Power Harvesting Systems,” and patent application Ser.No. 11/951,562 titled “Battery Power Delivery Module,” that are filed inat the U.S. Patent and Trademark Office on Dec. 4, 2007 and incorporatesthe entire content of these applications by this reference.

BACKGROUND

1. Field of the Invention

The field of the invention relates generally to power production fromdistributed DC power sources, and more particularly to management ofdistributed DC power sources in series installations.

2. Related Arts

The recent increased interest in renewable energy has led to increasedresearch in systems for distributed generation of energy, such asphotovoltaic cells (PV), fuel cells, batteries (e.g., for hybrid cars),etc. Various topologies have been proposed for connecting these powersources to the load, taking into consideration various parameters, suchas voltage/current requirements, operating conditions, reliability,safety, costs, etc. For example, most of these sources provide lowvoltage output (normally a few volts for one cell, or a few tens ofvolts for serially connected cells), so that many of them need to beconnected serially to achieve the required operating voltage.Conversely, a serial connection may fail to provide the requiredcurrent, so that several strings of serial connections may need to beconnected in parallel to provide the required current.

It is also known that power generation from each of these sourcesdepends on manufacturing, operating, and environmental conditions. Forexample, various inconsistencies in manufacturing may cause twoidentical sources to provide different output characteristics.Similarly, two identical sources may react differently to operatingand/or environmental conditions, such as load, temperature, etc. Inpractical installations, different source may also experience differentenvironmental conditions, e.g., in solar power installations some panelsmay be exposed to full sun, while others be shaded, thereby deliveringdifferent power output. In a multiple-battery installation, some of thebatteries may age differently, thereby delivering different poweroutput. While these problems and the solutions provided by the subjectinvention are applicable to any distributed power system, the followingdiscussion turns to solar energy so as to provide better understandingby way of a concrete example.

A conventional installation of solar power system 10 is illustrated inFIG. 1. Since the voltage provided by each individual solar panel 101 islow, several panels are connected in series to form a string of panels103. For a large installation, when higher current is required, severalstrings 103 may be connected in parallel to form the overall system 10.The solar panels are mounted outdoors, and their leads are connected toa maximum power point tracking (MPPT) module 107 and then to an inverter104. The MPPT 107 is typically implemented as part of the inverter 104.The harvested power from the DC sources is delivered to the inverter104, which converts the fluctuating direct-current (DC) intoalternating-current (AC) having a desired voltage and frequency, whichis usually 110V or 220V at 60 Hz, or 220V at 50 Hz (It is interesting tonote the even in the US many inverters produce 220V, which is then splitinto two 110V feeds in the electric box). The AC current from theinverter 104 may then be used for operating electric appliances or fedto the power grid. Alternatively, if the installation is not tied to thegrid, the power extracted from the inverter may be directed to aconversion and charge/discharge circuit to store the excess powercreated as charge in batteries. In case of a battery-tied application,the inversion stage might be skipped altogether, and the DC output ofthe MPPT stage 107 may be fed into the charge/discharge circuit.

As noted above, each solar panel 101 supplies relatively very lowvoltage and current. The problem facing the solar array designer is toproduce a standard AC current at 120V or 220V root-mean-square (RMS)from a combination of the low voltages of the solar panels. The deliveryof high power from a low voltage requires very high currents, whichcause large conduction losses on the order of the second power of thecurrent (I²). Furthermore, a power inverter, such as the inverter 104,which is used to convert DC current to AC current, is most efficientwhen its input voltage is slightly higher than its output RMS voltagemultiplied by the square root of 2. Hence, in many applications, thepower sources, such as the solar panels 101, are combined in order toreach the correct voltage or current. The most common method connectsthe power sources in series in order to reach the desirable voltage andin parallel in order to reach the desirable current, as shown in FIG. 1.A large number of the panels 101 are connected into a string 103 and thestrings 103 are connected in parallel to the power inverter 104. Thepanels 101 are connected in series in order to reach the minimal voltagerequired for the inverter. Multiple strings 103 are connected inparallel into an array to supply higher current, so as to enable higherpower output.

While this configuration is advantageous in terms of cost andarchitecture simplicity, several drawbacks have been identified in theliterature for such architecture. One recognized drawback isinefficiencies cause by non-optimal power draw from each individualpanel, as explained below. As explained above, the output of the DCpower sources is influenced by many conditions. Therefore, to maximizethe power draw from each source, one needs to draw the combination ofvoltage and current that provides the peak power for the currentlyprevailing conditions. As conditions change, the combination of voltageand current draw may need to be changed as well.

FIG. 2 illustrates one serial string of DC sources, e.g., solar panels201 a-201 d, connected to MPPT circuit 207 and inverter 204. The currentversus voltage (IV) characteristics plotted (210 a-210 d) to the left ofeach DC source 201. For each DC source 201, the current decreases as theoutput voltage increases. At some voltage value the current goes tozero, and in some applications may assume a negative value, meaning thatthe source becomes a sink. Bypass diodes are used to prevent the sourcefrom becoming a sink. The power output of each source 201, which isequal to the product of current and voltage (P=I*V), varies depending onthe voltage drawn from the source. At a certain current and voltage,close to the falling off point of the current, the power reaches itsmaximum. It is desirable to operate a power generating cell at thismaximum power point. The purpose of the MPPT is to find this point andoperate the system at this point so as to draw the maximum power fromthe sources.

In a typical, conventional solar panel array, different algorithms andtechniques are used to optimize the integrated power output of thesystem 10 using the MPPT module 107. The MPPT module 107 receives thecurrent extracted from all of the solar panels together and tracks themaximum power point for this current to provide the maximum averagepower such that if more current is extracted, the average voltage fromthe panels starts to drop, thus lowering the harvested power. The MPPTmodule 107 maintains a current that yields the maximum average powerfrom the overall system 10.

Maximum power point tracking techniques are reviewed in: “Comparison ofPhotovoltaic Array Maximum Power Point Tracking Techniques” by T. Esram& P. L. Chapman, IEEE Transactions on Energy Conversion (Accepted forfuture publication, Volume PP, Issue 99, 2006 Page(s):1-1, DigitalObject Identifier 10.1109/TEC.2006.874230), the entire content of whichis incorporated herein by this reference.

However, since the sources 201 a-201 d are connected in series to asingle MPPT 207, the MPPT must select a single point, which would besomewhat of an average of the MPP of the serially connected sources. Inpractice, it is very likely that the MPPT would operate at an I-V pointthat is optimum to only a few or none of the sources. In the example ofFIG. 2, the selected point is the maximum power point for source 201 b,but is off the maximum power point for sources 201 a, 201 c and 201 d.Consequently, the arrangement is not operated at best achievableefficiency.

Turning back to the example of a solar system 10 of FIG. 1, fixing apredetermined constant output voltage from the strings 103 may cause thesolar panels to supply lower output power than otherwise possible.Further, each string carries a single current that is passed through allof the solar panels along the string. If the solar panels are mismatcheddue to manufacturing differences, aging or if they malfunction or areplaced under different shading conditions, the current, voltage andpower output of each panel will be different. Forcing a single currentthrough all of the panels of the string causes the individual panels towork at a non-optimal power point and can also cause panels which arehighly mismatched to generate “hot spots” due to the high currentflowing through them. Due to these and other drawbacks of conventionalcentralized methods, the solar panels have to be matched properly. Insome cases external diodes are used to bypass the panels that are highlymismatched. In conventional multiple string configurations all stringshave to be composed of exactly the same number of solar panels and thepanels are selected of the same model and must be install at exactly thesame spatial orientation, being exposed to the same sunlight conditionsat all times. This is difficult to achieve and can be very costly.

Various different topologies have been proposed in order to overcome theabove deficiencies of the serial installation. For example, some haveproposed to have inverters coupled to each DC source, and connect all ofthe inverters in parallel. Others have proposed to have DC/DC converterconnected to each DC source, and to connect all of the convertersserially or in parallel to a central inverter. Among the DC/DCconverters proposed for use with the DC sources are boost converter,buck converter, buck-boost converter, or a Cuk converter. It has alsobeen proposed to incorporate MPPT into each DC power source, e.g., intoeach solar panel, and connect the panels serially.

For further discussion of the above issues relating to distributed powersources and solar panels, the reader is highly encouraged to review thefollowing literature, which may or may not be prior art.

-   Cascade DC-DC Converter Connection of Photovoltaic Modules, G. R.    Walker and P. C. Sernia, Power Electronics Specialists    Conference, 2002. (PESCO2), Vol. 1 IEEE, Cairns, Australia, pp.    24-29.-   Topology for Decentralized Solar Energy Inverters with a Low Voltage    AC-Bus, Bjorn Lindgren.-   Integrated Photovoltaic Maximum Power Point Tracking Converter,    Johan H. R. Enslin et al., IEEE Transactions on Industrial    Electronics, Vol. 44, No. 6, December 1997.-   A New Distributed Converter Interface for PV Panels, R. Alonso et    al., 20^(th) European Photovoltaic Solar Energy Conference, 6-10    Jun. 2005, Barcelona, Spain.    -   Intelligent PV Module for Grid-Connected PV Systems, Eduardo        Roman, et al., IEEE Transactions on Industrial Electronics, Vol.        53, No. 4, August 2006. Also in Spanish patent application        ES2249147.-   A Modular Fuel Cell, Modular DC-DC Converter Concept for High    Performance and Enhanced Reliability, L. Palma and P. Enjeti, Power    Electronics Specialists Conference, 2007, PESC 2007, IEEE Volume,    Issue, 17-21 Jun. 2007 Page(s):2633-2638. Digital Object Identifier    10.1109/PESC.2007.4342432.-   Experimental Results of Intelligent PV Module for Grid-Connected PV    Systems, R. Alonso et al., Twentyfirst European Photovoltaic Solar    Energy Conference, Proceedings of the International Conference held    in Dresden, Germany, 4-8 Sep. 2006.-   Cascaded DC-DC Converter Connection of Photovoltaic Modules, G. R.    Walker and P. C. Sernia, IEEE Transactions on Power Electronics,    Vol. 19, No. 4, July 2004.-   Cost Effectiveness of Shadow Tolerant Photovoltaic Systems,    Quaschning, V.; Piske, R.; Hanitsch, R., Euronsun 96, Freiburg, Sep.    16-19, 1996.-   Evaluation Test results of a New Distributed MPPT Converter, R.    Orduz and M. A. Egido, 22nd European Photovoltaic Solar Energy    Conference, 3-7 Sep. 2007, Milan, Italy.-   Energy Integrated Management System for PV Applications, S. Uriarte    et al., 20th European Photovoltaic Solar Energy Conference, 6-10    Jun. 2005, Barcelona, Spain.-   U.S. Published Application 2006/0185727

As noted in some of the above cited works, integrating inverters intothe individual cells has many drawbacks, including high costs, lowsafety (especially in solar installations), and reliability. Therefore,serial connection is still preferred, especially for solar panelinstallations. The proposals for including DC-DC converters and MPPTinto the individual sources, and then connect their outputs serially toan inverter are attractive. However, incorporating MPPT into each panelis still problematic in serial application, as each MPPT would attemptto drive its source at different current, while in a serial connectionthe same current must flow through all of the panels. Moreover, it isunclear what type of DC-DC converter would provide the best results andhow to incorporate an MPPT into such an arrangement. Therefore,solutions are still needed for an effective topology for connectingmultiple DC power sources to the load, i.e., power grid, power storagebank, etc.

As already mentioned above, various environmental and operationalconditions impact the power output of DC power sources. In the case ofsolar panels, solar radiance, ambient temperature, and shading, whetherfrom near objects such as trees or far objects such as clouds, impactthe power extracted from each solar panel. Depending on the number andtype of panels used, the extracted power may vary widely in the voltageand current. Owners and even professional installers find it difficultto verify the correct operation of the solar system. With time, manyother factors, such as aging, dust and dirt collection and moduledegradation affect the performance of the solar array.

The sensitivity of photovoltaic panels to external conditions is evenmore profound when concentrated phovoltaics (CPV) are used. In suchinstallations, the sun radiation is concentrated by use of lenses ormirrors onto small cells. These cells may be much more efficient thentypical PV cells and use a technology knows as double- ortriple-junction, in which a number of p-n junctions are constructed oneon top of the other—each junction converts light from a certain part ofthe spectrum and allows the rest to pass-through to the next junction.Thus, these cells are much more efficient (with peak efficiencies ofover 40%). Since these cells are expensive, they are usually used in CPVapplications which call for smaller cells. However, the power output ofCPV installations now depends upon fluctuations in the intensity ofdifferent parts of the spectrum of the sun (and not only the totalintensity), and imperfections or distortions in the lenses or mirrorsused. Thus, having a single MPPT for many panels will lead tosignificant power loss, and great benefits are realized from using apanel- (or cell-) level MPPT as described in aspects of the presentinvention.

Another field in which traditional photovoltaic installations face manyproblems is the developing market of building-integrated photovoltaics(BIPV). In BIPV installations, the panels are integrated into buildingsduring construction—either as roof panels or as structural or additionalelements in the walls and windows. Thus, BIPV installations suffergreatly from local partial shading due to the existence of otherstructural elements in the vicinity of the panels. Moreover, the panelsare naturally positioned on many different facets of the building, andtherefore the lighting conditions each panel experiences may varygreatly. Since in traditional solutions the panels are stringed togetherto a joint MPPT, much power is lost. A solution that could harvest morepower would obviously be very beneficial in installations of this type.

Yet another problem with traditional installations is the poor energyutilization in cases of low sun-light. Most inverters require a certainminimal voltage (typically between 150V to 350V) in order to startfunctioning. If there is low light, the aggregated voltage from thepanels may not reach this minimal value, and the power is thus lost. Asolution that could boost the voltage of panels suffering from lowlight, would therefore allow for the produced energy to be harvested.

During installation of a solar array according to the conventionalconfigurations 10, the installer can verify the correctness of theinstallation and performance of the solar array by using test equipmentto check the current-voltage characteristics of each panel, each stringand the entire array. In practice, however, individual panels andstrings are generally either not tested at all or tested only prior toconnection. This happens because current measurement is done by either aseries connection to the solar array or a series resistor in the arraywhich is typically not convenient. Instead, only high-level pass/failtesting of the overall installation is performed.

After the initial testing of the installation, the solar array isconnected to inverter 104 which optionally includes a monitoring modulewhich monitors performance of the entire array. The performanceinformation gathered from monitoring within the inverter 104 includesintegrated power output of the array and the power production rate, butthe information lacks any fine details about the functioning ofindividual solar panels. Therefore, the performance information providedby monitoring at the inverter 104 is usually not sufficient tounderstand if power loss is due to environmental conditions, frommalfunctions or from poor installation or maintenance of the solararray. Furthermore, integrated information does not pinpoint which ofsolar panels 101 is responsible for a detected power loss.

In view of the above, a newly proposed topology for connecting multipleDC power sources to the load should also lend itself to easy testing andoperational verification during and after installation.

SUMMARY

The following summary of the invention is provided in order to provide abasic understanding of some aspects and features of the invention. Thissummary is not an extensive overview of the invention, and as such it isnot intended to particularly identify key or critical elements of theinvention, or to delineate the scope of the invention. Its sole purposeis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented below.

Aspects of the invention provide a topology for distributed DC powersources serially connected to a central power supplier, e.g., a singleinverter or a single converter. Aspects of the invention provide systemand a method for monitoring of individual DC power sources in adistributed power harvesting installation and adjusting the current andvoltage from each DC power source to maximize power output from each DCpower source.

According to aspects of the invention, a distributed power harvestingsystem comprising: a plurality of DC power sources; a plurality ofconverters, each of the converters comprising: input terminals coupledto a respective DC power source; output terminals coupled in series tothe other converters, thereby forming a serial string; a circuit loopsetting the voltage and current at the input terminals of the converteraccording to predetermined criteria; and, a power conversion portion forconverting the power received at the input terminals to an output powerat the output terminals; and a power supplier coupled to the serialstring, the power supplier comprising a control part maintaining theinput to the power supplier at a predetermined value. The control partmay maintain the input voltage to the power supplier at a predeterminedvalue. The control part may maintain the input current to the powersupplier at a predetermined value. The power supplier may comprise aDC/AC inverter. The power supplier may comprise a battery charger. Thecircuit loop may comprise an MPPT part setting the voltage and currentat the input terminals of the converter to maximum power point of therespective DC power source. The power conversion portion may comprise: abuck converter; a boost converter; a controller selectively activatingeither the buck converter or the boost converter in response to the MPPTpart and current or voltage at the output terminals. An inductor may beshared by the buck converter and the boost converter, and the controllercomprises a pulse-width modulation portion. The control part maycomprise a shunt regulator coupled in parallel with the power supplierand regulating the input voltage to a preselected constant inputvoltage. The system may further comprise one or more additional serialstrings coupled to the power supplier. The system may further comprise:a plurality of current sensors; and a plurality of voltage sensors;wherein each of the current sensors and each of the voltage sensors iscoupled between a respective converter and DC power source and providingcurrent information and voltage information to the MPPT part. Each ofthe plurality of DC power sources may comprise a solar panel or abuilding integrated solar panel. At least one of the plurality of DCpower sources may comprise a fuel cell. At least one of the plurality ofDC power sources may comprise a battery. Each of the plurality ofconverters may further comprise a safety module limiting the output to apreset safe value until a predetermined event has occurred. Thepredetermined event may comprise one of a load above a preset thresholdis applied to the converter or a release signal has been detected. Eachof the converters may further comprise a plurality of switching devices,each of the switching devices forming a current bypass to at least oneDC power source. The solar panel may comprise a plurality of cellstrings, each cell string comprising serially connected solar cells anda switching device coupled to bypass the serially connected solar cells.The switching device may comprise a transistor. Each of the convertersmay further comprise a monitoring module monitoring and transmittingstatus related data, the status related data comprising at least one of:input current to the converter, input voltage to the converter,temperature of the power source, input power to the converter, andavailable illumination.

According to an aspect of the invention, a method for harvesting powerfrom a distributed power system having a plurality of DC power sourcesand a plurality of DC power converters is provided, the methodcomprising: coupling each of the power sources to a respective DC powerconverter; coupling the power converters in series, to thereby form atleast one serial string; coupling the serial string to a power deliverydevice; fixing one of input voltage or input current to the powerdelivery device to a predetermined value, thereby forcing currentflowing through the serial string to vary according to power provided bythe power sources; and controlling power output from each power sourceindividually and individually varying the input voltage and current toeach respective converter according to a predetermined criteria. Fixingone of the input voltage or input current may comprise fixing to apredetermined constant value. Coupling the serial string to a powerdelivery device may comprise coupling the serial string to a DC/ACinverter and fixing the input voltage to the inverter. Monitoring poweroutput may comprise tracking maximum power point of the power source,and individually varying the input voltage and current comprises settingthe input voltage and current so as to draw maximum power from eachpower source. The method may further comprise individually convertingthe input voltage and current of each converter to output power atcurrent level dictated by the current flowing through the serial stringand at a floating voltage. The method may further comprise individuallyconverting the input voltage and current of each converter to outputpower at current level dictated by the current flowing through theserial string and at a floating voltage. The method may further comprisemonitoring load on each converter individually and limiting power outputfrom each converter to a preset safe level until the load reaches apreset value. The method may further comprise monitoring power output ofat least one of the power source and DC power converter and directingcurrent to a bypass when the power output exhibits predeterminedcharacteristics. The method may further comprise individually operatingeach power converter to monitor and report power related data, the powerrelated data comprising at least one of: input current to the converter,input voltage to the converter, temperature of the power source, inputpower to the converter, and available illumination.

According to aspects of the invention, a solar power installation isprovided, comprising: a DC/AC inverter comprising means for maintainingthe input voltage or current to the inverter at a predetermined value; aplurality of serial strings coupled in parallel to the DC/AC inverter,each of the serial string comprising: a plurality of solar panels; aplurality of converters, each of the converters comprising: inputterminals coupled to a respective solar panel; output terminals coupledin series to the other converters, thereby forming one serial string; anMPPT part setting the voltage and current at the input terminals of theconverter according to maximum power point of the respective solarpanel; and, a power conversion portion for converting the power receivedat the input terminals to an output power at the output terminals. Thepredetermined value may comprise a constant value. The power conversionportion may convert the power received at the input terminals to outputpower having current substantially equal to the total power provided bythe plurality of solar panels in the serial string divided by thepredetermined constant voltage at the input of the inverter. The powerconversion portion may comprise a power conversion controllercontrolling pulse width modulation of the power conversion portion so asto output power having current substantially equal to the total powerprovided by the plurality of solar panels in the serial string dividedby the predetermined constant voltage at the input of the inverter. Eachof the power conversion portion may comprise: a buck converter; a boostconverter; a pulse width modulator; and, a digital controllercontrolling the pulse width modulator to selectively operate either thebuck converter or the boost converter. Each of the serial strings mayfurther comprise: a plurality of current sensors, each measuring currentoutput of one solar panel and sending measured current signal to arespective digital controller; and a plurality of voltage sensors, eachmeasuring voltage output of one solar panel and sending measured voltagesignal to a respective digital controller; wherein each digitalcontroller adjusts current and voltage draw to obtain maximum availablepower. The solar power installation may further comprise a safety modulelimiting the output voltage to a preset safe value as long as no loadabove a preset threshold is applied to the converter. The solar powerinstallation of claim 30, wherein each of the solar panels comprises aplurality of cell strings, each cell string comprising seriallyconnected solar cells and a switching device coupled to bypass theserially connected solar cells. The switching device may comprise atransistor. Each of the converters may further comprise a monitoringmodule monitoring and transmitting power related data, the power relateddata comprising at least one of: input current to the converter, inputvoltage to the converter, temperature of the power source, spatialorientation of the power source, and available illumination.

According to aspects of the invention, a method for improving thereliability of components within the load in a distributed power systemhaving a plurality of DC power sources coupled to a central load isprovided, comprising: coupling the DC power sources to the central load;maintaining the input to the central load to a fixed predeterminedvoltage, the voltage being a safe operating voltage for the componentswithin the load; varying the current input to the central load accordingto the power drawn from the DC power sources. The central load maycomprise a DC/AC inverter, and the step of maintaining the inputcomprises maintaining the input voltage to the inverter. Coupling the DCpower sources may comprise coupling each of a plurality of solar panelsto a respective converter from a plurality of converters, and couplingall of the converters to the inverter. The method may further compriseoperating each converter to boost the voltage obtained from a respectivesolar panel as soon as the respective solar panel starts to outputelectrical energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify the embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate major features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

FIG. 1 illustrates a conventional centralized power harvesting systemusing DC power sources.

FIG. 2 illustrates current versus voltage characteristic curves for oneserial string of DC sources.

FIG. 3 illustrates a distributed power harvesting system, according toaspects of the invention, using DC power sources.

FIGS. 4A and 4B illustrate the operation of the system of FIG. 3 underdifferent conditions, according to aspects of the invention.

FIG. 4C illustrates an embodiment of the invention wherein the invertercontrols the input current.

FIG. 5 illustrates a distributed power harvesting system, according toother aspects of the invention, using DC power sources.

FIG. 6 illustrates an exemplary DC-to-DC converter according to aspectsof the invention.

FIG. 7 illustrates a power converter, according to aspects of theinvention including control features of the aspects of the invention.

FIG. 8 illustrates an arrangement of a solar panel according to theprior art.

FIG. 9 illustrates an arrangement according to an embodiment of theinvention for reducing the power loss in solar strings.

FIG. 10 illustrates another arrangement according to an embodiment ofthe invention for reducing the power loss in solar strings.

FIG. 11 illustrates an arrangement according to an embodiment of theinvention for bypassing a solar string.

DETAILED DESCRIPTION

The topology provided by the subject invention solves many of theproblems associated with, and has many advantages over, the prior arttopologies. For example, the inventive topology enables seriallyconnecting mismatched power sources, such as mismatched solar panels,panel of different models and power ratings, and even panels fromdifferent manufacturers and semiconductor materials. It allows serialconnection of sources operating under different conditions, such as,e.g., solar panels exposed to different light or temperature conditions.It also enables installations of serially connected panels at differentorientations or different sections of the roof or structure. This andother features and advantages will become apparent from the followingdetailed description.

Aspects of the present invention provide a system and method forcombining power from multiple DC power sources into a single powersupply. According to aspects of the present invention, each DC powersource is associated with a DC-DC power converter. Modules formed bycoupling the DC power sources to their associated converters are coupledin series to provide a string of modules. The string of modules is thencoupled to an inverter having its input voltage fixed. A maximum powerpoint control loop in each converter harvests the maximum power fromeach DC power source and transfers this power as output from the powerconverter. For each converter, substantially all the input power isconverted to the output power, such that the conversion efficiency maybe 90% or higher in some situations. Further, the controlling isperformed by fixing the input current or input voltage of the converterto the maximum power point and allowing output voltage of the converterto vary. For each power source, one or more sensors perform themonitoring of the input power level to the associated converter. In someaspects of the invention, a microcontroller may perform the maximumpower point tracking and control in each converter by using pulse widthmodulation to adjust the duty cycle used for transferring power from theinput to the output.

One aspect of the present invention provides a greater degree of faulttolerance, maintenance and serviceability by monitoring, logging and/orcommunicating the performance of each solar panel. In one aspect of theinvention, the microcontroller that is used for maximum power pointtracking, may also be used to perform the monitoring, logging andcommunication functions. These functions allow for quick and easytroubleshooting during installation, thereby significantly reducinginstallation time. These functions are also beneficial for quickdetection of problems during maintenance work. Aspects of the presentinvention allow easy location, repair, or replacement of failed solarpanels. When repair or replacement is not feasible, bypass features ofthe current invention provide increased reliability.

In one aspect, the present invention relates to arrays of solar cellswhere the power from the cells is combined. Each converter may beattached to a single solar cell, or a plurality of cell connected inseries, in parallel, or both, e.g., parallel connection of strings ofserially connected cells. In one embodiment each converter is attachedto one panel of photovoltaic strings. However, while applicable in thecontext of solar power technology, the aspects of the present inventionmay be used in any distributed power network using DC power sources. Forexample, they may be used in batteries with numerous cells or hybridvehicles with multiple fuel cells on board. The DC power sources may besolar cells, solar panels, electrical fuel cells, electrical batteries,and the like. Further, although the discussion below relates tocombining power from an array of DC power sources into a source of ACvoltage, the aspects of the present invention may also apply tocombining power from DC sources into another DC voltage.

FIG. 3 illustrates a distributed power harvesting configuration 30,according to an embodiment of the present invention. Configuration 30enables connection of multiple power sources, for example solar panels301 a-301 d, to a single power supply. In one aspect of the invention,the series string of all of the solar panels may be coupled to aninverter 304. In another aspect of the invention, several seriallyconnected strings of solar panels may be connected to a single inverter304. The inverter 304 may be replaced by other elements, such as, e.g.,a charging regulator for charging a battery bank.

In configuration 30, each solar panel 301 a-301 d is connected to aseparate power converter circuit 305 a-305 d. One solar panel togetherwith its associated power converter circuit forms a module, e.g., module320. Each converter 305 a-305 d adapts optimally to the powercharacteristics of the connected solar panel 301 a-301 d and transfersthe power efficiently from converter input to converter output. Theconverters 305 a-305 d can be buck converters, boost converters,buck/boost converters, flyback or forward converters, etc. Theconverters 305 a-305 d may also contain a number of componentconverters, for example a serial connection of a buck and a boostconverter.

Each converter 305 a-305 d includes a control loop that receives afeedback signal, not from the converter's output current or voltage, butrather from the converter's input coming from the solar panel 301. Anexample of such a control loop is a maximum power point tracking (MPPT)loop. The MPPT loop in the converter locks the input voltage and currentfrom each solar panel 301 a-301 d to its optimal power point.

Conventional DC-to-DC converters may have a wide input voltage range attheir input and an output voltage that is predetermined and fixed. Inthese conventional DC-to-DC voltage converters, a controller within theconverter monitors the current or voltage at the input, and the voltageat the output. The controller determines the appropriate pulse widthmodulation (PWM) duty cycle to fix the output voltage to thepredetermined value by increasing the duty cycle if the output voltagedrops. Accordingly, the conventional converter includes a feedback loopthat closes on the output voltage and uses the output voltage to furtheradjust and fine tune the output voltage from the converter. As a resultof changing the output voltage, the current extracted from the input isalso varied.

In the converters 305 a-305 d, according to aspects of the presentinvention, a controller within the converter 405 monitors the voltageand current at the converter input and determines the PWM in such a waythat maximum power is extracted from the attached panel 301 a-301 d. Thecontroller of the converter 405 dynamically tracks the maximum powerpoint at the converter input. In the aspects of the present invention,the feedback loop is closed on the input power in order to track maximuminput power rather than closing the feedback loop on the output voltageas performed by conventional DC-to-DC voltage converters.

As a result of having a separate MPPT circuit in each converter 305a-305 d, and consequently for each solar panel 301 a-301 d, each string303 in the embodiment shown in FIG. 3 may have a different number ordifferent brand of panels 301 a-301 d connected in series. The circuitof FIG. 3 continuously performs MPPT on the output of each solar panel301 a-301 d to react to changes in temperature, solar radiance, shadingor other performance factors that impact that particular solar panel 301a-301 d. As a result, the MPPT circuit within the converters 305 a-305 dharvests the maximum possible power from each panel 301 a-301 d andtransfers this power as output regardless of the parameters impactingthe other solar panels.

As such, the aspects of the invention shown in FIG. 3 continuously trackand maintain the input current and the input voltage to each converterat the maximum power point of the DC power source providing the inputcurrent and the input voltage to the converter. The maximum power of theDC power source that is input to the converter is also output from theconverter. The converter output power may be at a current and voltagedifferent from the converter input current and voltage. The outputcurrent and voltage from the converter are responsive to requirements ofthe series connected portion of the circuit.

In one aspect of the invention, the outputs of converters 305 a-305 dare series connected into a single DC output that forms the input to theload or power supplier, in this example, inverter 304. The inverter 304converts the series connected DC output of the converters into an ACpower supply. The load, in this case inverter 304, regulates the voltageat the load's input. That is, in this example, an independent controlloop 320 holds the input voltage at a set value, say 400 volts.Consequently, the inverter's input current is dictated by the availablepower, and this is the current that flows through all serially connectedDC sources. On the other hand, while the output of the DC-DC convertersmust be at the inverter's current input, the current and voltage inputto the converter is independently controlled using the MPPT.

In the prior art, the input voltage to the load was allowed to varyaccording to the available power. For example, when a lot of sunshine isavailable in a solar installation, the voltage input to the inverter canvary even up to 1000 volts. Consequently, as sunshine illuminationvaries, the voltage varies with it, and the electrical components in theinverter (or other power supplier or load) are exposed to varyingvoltage. This tends to degrade the performance of the components andultimately causes them to fail. On the other hand, by fixing the voltageor current to the input of the load or power supplier, here theinverter, the electrical components are always exposed to the samevoltage or current and therefore would have extended service life. Forexample, the components of the load (e.g., capacitors, switches and coilof the inverter) may be selected so that at the fixed input voltage orcurrent they operate at, say, 60% of their rating. This would improvethe reliability and prolong the service life of the component, which iscritical for avoiding loss of service in applications such as solarpower systems.

FIGS. 4A and 4B illustrate the operation of the system of FIG. 3 underdifferent conditions, according to aspects of the invention. Theexemplary configuration 40 is similar to configuration 30 of FIG. 3. Inthe example shown, ten DC power sources 401/1 through 401/10 areconnected to ten power converters 405/1 through 405/10, respectively.The modules formed by the DC power sources and their correspondingconverters are coupled together in series to form a string 403. In oneaspect of the invention, the series-connected converters 405 are coupledto a DC-to-AC inverter 404.

The DC power sources may be solar panels and the example is discussedwith respect to solar panels as one illustrative case. Each solar panel401 may have a different power output due to manufacturing tolerances,shading, or other factors. For the purpose of the present example, anideal case is illustrated in FIG. 4A, where efficiency of the DC-to-DCconversion is assumed to be 100% and the panels 501 are assumed to beidentical. In some aspects of the invention, efficiencies of theconverters may be quite high and range at about 95%-99%. So, theassumption of 100% efficiency is not unreasonable for illustrationpurposes. Moreover, according to embodiments of the subject invention,each of the DC-DC converters is constructed as a power converter, i.e.,it transfers to its output the entire power it receives in its inputwith very low losses.

Power output of each solar panel 401 is maintained at the maximum powerpoint for the panel by a control loop within the corresponding powerconverter 405. In the example shown in FIG. 4A, all of the panels areexposed to full sun illumination and each solar panel 401 provides 200 Wof power. Consequently, the MPPT loop will draw current and voltagelevel that will transfer the entire 200 W from the panel to itsassociated converter. That is, the current and voltage dictated by theMPPT form the input current I_(in) and input voltage V_(in) to theconverter. The output voltage is dictated by the constant voltage set atthe inverter 404, as will be explained below. The output current I_(out)would then be the total power, i.e., 200 W, divided by the outputvoltage V_(out).

As noted above, according to a feature of the invention, the inputvoltage to inverter 404 is controlled by the inverter (in this example,kept constant), by way of control loop 420. For the purpose of thisexample, assume the input voltage is kept as 400V (ideal value forinverting to 220 VAC). Since we assume that there are ten seriallyconnected power converters, each providing 200 W, we can see that theinput current to the inverter 404 is 2000 W/400V=5 A. Thus, the currentflowing through each of the converters 401/1-401/10 must be 5 A. Thismeans that in this idealized example each of the converters provides anoutput voltage of 200 W/5 A=40V. Now, assume that the MPPT for eachpanel (assuming perfect matching panels) dictates V_(MPP)=32V. Thismeans that the input voltage to the inverter would be 32V, and the inputcurrent would be 200 W/32V=6.25 A.

We now turn to another example, wherein the system is still maintainedat an ideal mode (i.e., perfectly matching DC sources and entire poweris transferred to the inverter), but the environmental conditions arenot ideal. For example, one DC source is overheating, is malfunctioning,or, as in the example of FIG. 4B, the ninth solar panel 401/9 is shadedand consequently produces only 40 W of power. Since we keep all otherconditions as in the example of FIG. 4A, the other nine solar panels 401are unshaded and still produce 200 W of power. The power converter 405/9includes MPPT to maintain the solar panel 501/9 operating at the maximumpower point, which is now lowered due to the shading.

The total power available from the string is now 9×200 W+40 W=1840 W.Since the input to the inverter is still maintained at 400V, the inputcurrent to the inverter will now be 1840 W/40V=4.6 A. This means thatthe output of all of the power converters 405/1-405/10 in the stringmust be at 4.6 A. Therefore, for the nine unshaded panels, theconverters will output 200 W/4.6 A=43.5V. On the other hand, theconverter 405/9 attached to the shaded panel 401/9 will output 40 W/4.6A=8.7V. Checking the math, the input to the inverter can be obtained byadding nine converters providing 43.5V and one converter providing 8.7V,i.e., (9×43.5V)+8.7V=400V.

The output of the nine non-shaded panels would still be controlled bythe MPPT as in FIG. 4A, thereby standing at 32V and 6.25 A. On the otherhand, since the nines panel 401/9 is shaded, lets assume its MPPTdropped to 28V. Consequently, the output current of the ninth panel is40 W/28V=1.43 A. As can be seen by this example, all of the panels areoperated at their maximum power point, regardless of operatingconditions. As shown by the example of FIG. 4B, even if the output ofone DC source drops dramatically, the system still maintains relativelyhigh power output by fixing the voltage input to the inverter, andcontrolling the input to the converters independently so as to drawpower from the DC source at the MPP.

As can be appreciated, the benefit of the topology illustrated in FIGS.4A and 4B are numerous. For example, the output characteristics of theserially connected DC sources, such as solar panels, need not match.Consequently, the serial string may utilize panels from differentmanufacturers or panels installed on different parts of the roofs (i.e.,at different spatial orientation). Moreover, if several strings areconnected in parallel, it is not necessary that the strings match,rather each string may have different panels or different number ofpanels. This topology also enhances reliability by alleviating the hotspot problem. That is, as shown in FIG. 4A the output of the shadedpanel 401/9 is 1.43 A, while the current at the output of the unshadedpanels is 6.25 A. This discrepancy in current when the components areseries connected causes a large current being forced through the shadedpanel that may cause overheating and malfunction at this component.However, by the inventive topology wherein the input voltage is setindependently, and the power draw from each panel to its converter isset independently according to the panels MPP at each point in time, thecurrent at each panel is independent on the current draw from theserially connected converters.

It is easily realized that since the power is optimized independentlyfor each panel, panels could be installed in different facets anddirections in BIPV installations. Thus, the problem of low powerutilization in building-integrated installations is solved, and moreinstallations may now be profitable.

The described system could also easily solve the problem of energyharvesting in low light conditions. Even small amounts of light areenough to make the converters 405 operational, and they then starttransferring power to the inverter. If small amounts of power areavailable, there will be a low current flow—but the voltage will be highenough for the inverter to function, and the power will indeed beharvested.

According to aspects of the invention, the inverter 404 includes acontrol loop 420 to maintain an optimal voltage at the input of inverter404. In the example of FIG. 4B, the input voltage to inverter 404 ismaintained at 400V by the control loop 420. The converters 405 aretransferring substantially all of the available power from the solarpanels to the input of the inverter 404. As a result, the input currentto the inverter 404 is dependent only on the power provided by the solarpanels and the regulated set, i.e., constant, voltage at the inverterinput.

The conventional inverter 104, shown in FIG. 1 and FIG. 3A, is requiredto have a very wide input voltage to accommodate for changingconditions, for example a change in luminance, temperature and aging ofthe solar array. This is in contrast to the inverter 404 that isdesigned according to aspects of the present invention. The inverter 404does not require a wide input voltage and is therefore simpler to designand more reliable. This higher reliability is achieved, among otherfactors, by the fact that there are no voltage spikes at the input tothe inverter and thus the components of the inverter experience lowerelectrical stress and may last longer.

When the inverter 404 is a part of the circuit, the power from thepanels is transferred to a load that may be connected to the inverter.To enable the inverter 404 to work at its optimal input voltage, anyexcess power produced by the solar array, and not used by the load, isdissipated. Excess power may be handled by selling the excess power tothe utility company if such an option is available. For off-grid solararrays, the excess power may be stored in batteries. Yet another optionis to connect a number of adjacent houses together to form a micro-gridand to allow load-balancing of power between the houses. If the excesspower available from the solar array is not stored or sold, then anothermechanism may be provided to dissipate excess power.

The features and benefits explained with respect to FIGS. 4A and 4Bstem, at least partially, from having the inverter dictates the voltageprovided at its input. Conversely, a design can be implemented whereinthe inverter dictates the current at its input. Such an arrangement isillustrated in FIG. 4C. FIG. 4C illustrates an embodiment of theinvention wherein the inverter controls the input current. Power outputof each solar panel 401 is maintained at the maximum power point for thepanel by a control loop within the corresponding power converter 405. Inthe example shown in FIG. 4C, all of the panels are exposed to full sunillumination and each solar panel 401 provides 200 W of power.Consequently, the MPPT loop will draw current and voltage level thatwill transfer the entire 200 W from the panel to its associatedconverter. That is, the current and voltage dictated by the MPPT formthe input current I_(in) and input voltage V_(in) to the converter. Theoutput voltage is dictated by the constant current set at the inverter404, as will be explained below. The output voltage V_(out) would thenbe the total power, i.e., 200 W, divided by the output current I_(out).

As noted above, according to a feature of the invention, the inputcurrent to inverter 404 is dictated by the inverter by way of controlloop 420. For the purpose of this example, assume the input current iskept as 5 A. Since we assume that there are ten serially connected powerconverters, each providing 200 W, we can see that the input voltage tothe inverter 404 is 2000 W/5 A=400V. Thus, the current flowing througheach of the converters 401/1-401/10 must be 5 A. This means that in thisidealized example each of the converters provides an output voltage of200 W/5 A=40V. Now, assume that the MPPT for each panel (assumingperfect matching panels) dictates V_(MPP)=32V. This means that the inputvoltage to the inverter would be 32V, and the input current would be 200W/32V=6.25 A.

Consequently, similar advantages have been achieved by having theinverter control the current, rather than the voltage. However, unlikethe prior art, changes in the output of the panels will not cause inchanges in the current flowing to the inverter, as that is dictated bythe inverter itself. Therefore, if the inverter is designed to keep thecurrent or the voltage constant, then regardless of the operation of thepanels, the current or voltage to the inverter will remain constant.

FIG. 5 illustrates a distributed power harvesting system, according toother aspects of the invention, using DC power sources. FIG. 5illustrates multiple strings 503 coupled together in parallel. Each ofthe strings is a series connection of multiple modules and each of themodules includes a DC power source 501 that is coupled to a converter505. The DC power source may be a solar panel. The output of theparallel connection of the strings 503 is connected, again in parallel,to a shunt regulator 506 and a load controller 504. The load controller504 may be an inverter as with the embodiments of FIGS. 4A and 4B. Shuntregulators automatically maintain a constant voltage across itsterminals. The shunt regulator 506 is configured to dissipate excesspower to maintain the input voltage at the input to the inverter 504 ata regulated level and prevent the inverter input voltage fromincreasing. The current which flows through shunt regulator 506complements the current drawn by inverter 504 in order to ensure thatthe input voltage of the inverter is maintained at a constant level, forexample at 400V.

By fixing the inverter input voltage, the inverter input current isvaried according to the available power draw. This current is dividedbetween the strings 503 of the series connected converters. When eachconverter includes a controller loop maintaining the converter inputvoltage at the maximum power point of the associated DC power source,the output power of the converter is determined. The converter power andthe converter output current together determine the converter outputvoltage. The converter output voltage is used by a power conversioncircuit in the converter for stepping up or stepping down the converterinput voltage to obtain the converter output voltage from the inputvoltage as determined by the MPPT.

FIG. 6 illustrates an exemplary DC-to-DC converter 605 according toaspects of the invention. DC-to-DC converters are conventionally used toeither step down or step up a varied or constant DC voltage input to ahigher or a lower constant voltage output, depending on the requirementsof the circuit. However, in the embodiment of FIG. 6 the DC-DC converteris used as a power converter, i.e., transferring the input power tooutput power, the input voltage varying according to the MPPT, while theoutput current being dictated by the constant input voltage to theinverter. That is, the input voltage and current may vary at any timeand the output voltage and current may vary at any time, depending onthe operating condition of the DC power sources.

The converter 605 is connected to a corresponding DC power source 601 atinput terminals 614 and 616. The converted power of the DC power source601 is output to the circuit through output terminals 610, 612. Betweenthe input terminals 614, 616 and the output terminals 610, 612, theremainder of the converter circuit is located that includes input andoutput capacitors 620, 640, backflow prevention diodes 622, 642 and apower conversion circuit including a controller 606 and an inductor 608.

The inputs 616 and 614 are separated by a capacitor 620 which acts as anopen to a DC voltage. The outputs 610 and 612 are also separated by acapacitor 640 that also acts an open to DC output voltage. Thesecapacitors are DC-blocking or AC-coupling capacitors that short whenfaced with alternating current of a frequency for which they areselected. Capacitor 640 coupled between the outputs 610, 612 and alsooperates as a part of the power conversion circuit discussed below.

Diode 642 is coupled between the outputs 610 and 612 with a polaritysuch that current may not backflow into the converter 605 from thepositive lead of the output 612. Diode 622 is coupled between thepositive output lead 612 through inductor 608 which acts a short for DCcurrent and the negative input lead 614 with such polarity to prevent acurrent from the output 612 to backflow into the solar panel 601.

The DC power sources 601 may be solar panels. A potential differenceexists between the wires 614 and 616 due to the electron-hole pairsproduced in the solar cells of panel 601. The converter 605 maintainsmaximum power output by extracting current from the solar panel 601 atits peak power point by continuously monitoring the current and voltageprovided by the panel and using a maximum power point trackingalgorithm. The controller 606 includes an MPPT circuit or algorithm forperforming the peak power tracking Peak power tracking and pulse widthmodulation, PWM, are performed together to achieve the desired inputvoltage and current. The MPPT in the controller 606 may be anyconventional MPPT, such as, e.g., perturb and observe (P&O), incrementalconductance, etc. However, notably the MPPT is performed on the paneldirectly, i.e., at the input to the converter, rather than at the outputof the converter. The generated power is then transferred to the outputterminals 610 and 612. The outputs of multiple converters 605 may beconnected in series, such that the positive lead 612 of one converter605 is connected to the negative lead 610 of the next converter 605.

In FIG. 6, the converter 605 is shown as a buck plus boost converter.The term “buck plus boost” as used herein is a buck converter directlyfollowed by a boost converter as shown in FIG. 6, which may also appearin the literature as “cascaded buck-boost converter”. If the voltage isto be lowered, the boost portion is substantially shorted. If thevoltage is to be raised, the buck portion is substantially shorted. Theterm “buck plus boost” differs from buck/boost topology which is aclassic topology that may be used when voltage is to be raised orlowered. The efficiency of “buck/boost” topology is inherently lowerthen a buck or a boost. Additionally, for given requirements, abuck-boost converter will need bigger passive components then a buckplus boost converter in order to function. Therefore, the buck plusboost topology of FIG. 6 has a higher efficiency than the buck/boosttopology. However, the circuit of FIG. 6 continuously decides whether itis bucking or boosting. In some situations when the desired outputvoltage is similar to the input voltage, then both the buck and boostportions may be operational.

The controller 606 may include a pulse width modulator, PWM, or adigital pulse width modulator, DPWM, to be used with the buck and boostconverter circuits. The controller 606 controls both the buck converterand the boost converter and determines whether a buck or a boostoperation is to be performed. In some circumstances both the buck andboost portions may operate together. That is, as explained with respectto the embodiments of FIGS. 4A and 4B, the input voltage and current areselected independently of the selection of output current and voltage.Moreover, the selection of either input or output values may change atany given moment depending on the operation of the DC power sources.Therefore, in the embodiment of FIG. 6 the converter is constructed sothat at any given time a selected value of input voltage and current maybe up converted or down converted depending on the output requirement.

In one implementation, an integrated circuit (IC) 604 may be used thatincorporates some of the functionality of converter 605. IC 604 isoptionally a single ASIC able to withstand harsh temperature extremespresent in outdoor solar installations. ASIC 604 may be designed for ahigh mean time between failures (MTBF) of more than 25 years. However, adiscrete solution using multiple integrated circuits may also be used ina similar manner. In the exemplary embodiment shown in FIG. 6, the buckplus boost portion of the converter 605 is implemented as the IC 604.Practical considerations may lead to other segmentations of the system.For example, in one aspect of the invention, the IC 604 may include twoICs, one analog IC which handles the high currents and voltages in thesystem, and one simple low-voltage digital IC which includes the controllogic. The analog IC may be implemented using power FETs which mayalternatively be implemented in discrete components, FET drivers, A/Ds,and the like. The digital IC may form the controller 606.

In the exemplary circuit shown, the buck converter includes the inputcapacitor 620, transistors 628 and 630 a diode 622 positioned inparallel to transistor 628, and an inductor 608. The transistors 628,630 each have a parasitic body diode 624, 626. In the exemplary circuitshown, the boost converter includes the inductor 608, which is sharedwith the buck converter, transistors 648 and 650 a diode 642 positionedin parallel to transistor 650, and the output capacitor 640. Thetransistors 648, 650 each have a parasitic body diode 644, 646.

As shown in FIG. 1, adding electronic elements in the series arrangementmay reduce the reliability of the system, because if one electricalcomponent breaks it may affect the entire system. Specifically, if afailure in one of the serially connected elements causes an open circuitin the failed element, current ceases to flow through the entire series,thereby causing the entire system to stop function. Aspects of thepresent invention provide a converter circuit where electrical elementsof the circuit have one or more bypass routes associated with them thatcarry the current in case of the electrical element fails. For example,each switching transistor of either the buck or the boost portion of theconverter has its own bypass. Upon failure of any of the switchingtransistors, that element of the circuit is bypassed. Also, uponinductor failure, the current bypasses the failed inductor through theparasitic diodes of the transistor used in the boost converter.

FIG. 7 illustrates a power converter, according to aspects of theinvention. FIG. 7 highlights, among others, a monitoring and controlfunctionality of a DC-to-DC converter 705, according to embodiments ofthe present invention. A DC voltage source 701 is also shown in thefigure. Portions of a simplified buck and boost converter circuit areshown for the converter 705. The portions shown include the switchingtransistors 728, 730, 748 and 750 and the common inductor 708. Each ofthe switching transistors is controlled by a power conversion controller706.

The power conversion controller 706 includes the pulse-width modulation(PWM) circuit 733, and a digital control machine 730 including aprotection portion 737. The power conversion controller 706 is coupledto microcontroller 790, which includes an MPPT module 719, and may alsooptionally include a communication module 709, a monitoring and loggingmodule 711, and a protection module 735.

A current sensor 703 may be coupled between the DC power source 701 andthe converter 705, and output of the current sensor 703 may be providedto the digital control machine 730 through an associated analog todigital converter 723. A voltage sensor 704 may be coupled between theDC power source 701 and the converter 705 and output of the voltagesensor 704 may be provided to the digital control machine 730 through anassociated analog to digital converter 724. The current sensor 703 andthe voltage sensor 704 are used to monitor current and voltage outputfrom the DC power source, e.g., the solar panel 701. The measuredcurrent and voltage are provided to the digital control machine 730 andare used to maintain the converter input power at the maximum powerpoint.

The PWM circuit 733 controls the switching transistors of the buck andboost portions of the converter circuit. The PWM circuit may be adigital pulse-width modulation (DPWM) circuit. Outputs of the converter705 taken at the inductor 708 and at the switching transistor 750 areprovided to the digital control machine 730 through analog to digitalconverters 741, 742, so as to control the PWM circuit 733.

A random access memory (RAM) module 715 and a non-volatile random accessmemory (NVRAM) module 713 may be located outside the microcontroller 790but coupled to the microcontroller 790. A temperature sensor 779 and oneor more external sensor interfaces 707 may be coupled to themicrocontroller 790. The temperature sensor 779 may be used to measurethe temperature of the DC power source 701. A physical interface 717 maybe coupled to the microcontroller 790 and used to convert data from themicrocontroller into a standard communication protocol and physicallayer. An internal power supply unit 739 may be included in theconverter 705.

In various aspects of the invention, the current sensor 703 may beimplemented by various techniques used to measure current. In one aspectof the invention, the current measurement module 703 is implementedusing a very low value resistor. The voltage across the resistor will beproportional to the current flowing through the resistor. In anotheraspect of the invention, the current measurement module 703 isimplemented using current probes which use the Hall Effect to measurethe current through a conductor without adding a series resistor. Aftertranslating the current to voltage, the data may be passed through a lowpass filter and then digitized. The analog to digital converterassociated with the current sensor 703 is shown as the A/D converter 723in FIG. 7. Aliasing effect in the resulting digital data may be avoidedby selecting an appropriate resolution and sample rate for the analog todigital converter. If the current sensing technique does not require aseries connection, then the current sensor 703 may be connected to theDC power source 701 in parallel.

In one aspect of the invention, the voltage sensor 704 uses simpleparallel voltage measurement techniques in order to measure the voltageoutput of the solar panel. The analog voltage is passed through a lowpass filter in order to minimize aliasing. The data is then digitizedusing an analog to digital converter. The analog to digital converterassociated with the voltage sensor 704 are shown as the A/D converter724 in FIG. 7. The A/D converter 724 has sufficient resolution togenerate an adequately sampled digital signal from the analog voltagemeasured at the DC power source 701 that may be a solar panel.

The current and voltage data collected for tracking the maximum powerpoint at the converter input may be used for monitoring purposes also.An analog to digital converter with sufficient resolution may correctlyevaluate the panel voltage and current. However, to evaluate the stateof the panel, even low sample rates may be sufficient. A low-pass filtermakes it possible for low sample rates to be sufficient for evaluatingthe state of the panel. The current and voltage date may be provided tothe monitoring and logging module 711 for analysis.

The temperature sensor 779 enables the system to use temperature data inthe analysis process. The temperature is indicative of some types offailures and problems. Furthermore, in the case that the power source isa solar panel, the panel temperature is a factor in power outputproduction.

The one or more optional external sensor interfaces 707 enableconnecting various external sensors to the converter 705. Externalsensors are optionally used to enhance analysis of the state of thesolar panel 701, or a string or an array formed by connecting the solarpanels 701. Examples of external sensors include ambient temperaturesensors, solar radiance sensors, and sensors from neighboring panels.External sensors may be integrated into the converter 705 instead ofbeing attached externally.

In one aspect of the invention, the information acquired from thecurrent and voltage sensors 703, 704 and the optional temperature andexternal sensors 705, 707 may be transmitted to a central analysisstation for monitoring, control, and analysis using the communicationsinterface 709. The central analysis station is not shown in the figure.The communication interface 709 connects a microcontroller 790 to acommunication bus. The communication bus can be implemented in severalways. In one aspect of the invention, the communication bus isimplemented using an off-the-shelf communication bus such as Ethernet orRS422. Other methods such as wireless communications or power linecommunications, which could be implemented on the power line connectingthe panels, may also be used. If bidirectional communication is used,the central analysis station may request the data collected by themicrocontroller 790. Alternatively or in addition, the informationacquired from sensors 703, 704, 705, 707 is logged locally using themonitoring and logging module 711 in local memory such as the RAM 715 orthe NVRAM 713.

Analysis of the information from sensors 703, 704, 705, 707 enablesdetection and location of many types of failures associated with powerloss in solar arrays. Smart analysis can also be used to suggestcorrective measures such as cleaning or replacing a specific portion ofthe solar array. Analysis of sensor information can also detect powerlosses caused by environmental conditions or installation mistakes andprevent costly and difficult solar array testing.

Consequently, in one aspect of the invention, the microcontroller 790simultaneously maintains the maximum power point of input power to theconverter 705 from the attached DC power source or solar panel 701 basedon the MPPT algorithm in the MPPT module 719 and manages the process ofgathering the information from sensors 703, 704, 705, 707. The collectedinformation may be stored in the local memory 713, 715 and transmittedto an external central analysis station. In one aspect of the invention,the microcontroller 790 uses previously defined parameters stored in theNVRAM 713 in order to operate. The information stored in the NVRAM 713may include information about the converter 705 such as serial number,the type of communication bus used, the status update rate and the ID ofthe central analysis station. This information may be added to theparameters collected by the sensors before transmission.

The converters 705 may be installed during the installation of the solararray or retrofitted to existing installations. In both cases, theconverters 705 may be connected to a panel junction connection box or tocables connecting the panels 701. Each converter 705 may be providedwith the connectors and cabling to enable easy installation andconnection to solar panels 701 and panel cables.

In one aspect of the invention, the physical interface 717 is used toconvert to a standard communication protocol and physical layer so thatduring installation and maintenance, the converter 705 may be connectedto one of various data terminals, such as a computer or PDA. Analysismay then be implemented as software which will be run on a standardcomputer, an embedded platform or a proprietary device.

The installation process of the converters 705 includes connecting eachconverter 705 to a solar panel 701. One or more of the sensors 703, 704,705, 707 may be used to ensure that the solar panel 701 and theconverter 705 are properly coupled together. During installation,parameters such as serial number, physical location and the arrayconnection topology may be stored in the NVRAM 713. These parameters maybe used by analysis software to detect future problems in solar panels701 and arrays.

When the DC power sources 701 are solar panels, one of the problemsfacing installers of photovoltaic solar panel arrays is safety. Thesolar panels 701 are connected in series during the day when there issunlight. Therefore, at the final stages of installation, when severalsolar panels 701 are connected in series, the voltage across a string ofpanels may reach dangerous levels. Voltages as high as 600V are commonin domestic installations. Thus, the installer faces a danger ofelectrocution. The converters 705 that are connected to the panels 701may use built-in functionality to prevent such a danger. For example,the converters 705 may include circuitry or hardware of software safetymodule that limits the output voltage to a safe level until apredetermined minimum load is detected. Only after detecting thispredetermined load, the microcontroller 790 ramps up the output voltagefrom the converter 705.

Another method of providing a safety mechanism is to use communicationsbetween the converters 705 and the associated inverter for the string orarray of panels. This communication, that may be for example a powerline communication, may provide a handshake before any significant orpotentially dangerous power level is made available. Thus, theconverters 705 would wait for an analog or digital release signal fromthe inverter in the associated array before transferring power toinverter.

The above methodology for monitoring, control and analysis of the DCpower sources 701 may be implemented on solar panels or on strings orarrays of solar panels or for other power sources such as batteries andfuel cells.

FIG. 8 illustrates an arrangement of a solar panel according to theprior art. In FIG. 8, solar panel 800 comprises solar cells 805, whichare grouped into serially connected strings 810. The strings 810 areconnected together in series. For each string 810, a bypass diode 820 isprovided so that in the event of drop in power output of one string,that string may be bypassed via the respective diode 820 instead ofhaving the cells enter a negative voltage region, which will lead topower dissipation across them and may cause them to burn. However, whencurrent flows through the diodes, they dissipate energy. For example, ifa current of 5 A flows through a conventional diode having 0.7 voltcut-in voltage, the loss is 3.5 W. In practice the loss may easilyamount to 10 W.

FIG. 9 illustrates an arrangement according to an embodiment of theinvention for reducing the power loss in solar strings. In FIG. 9, thesolar panel 900 is made of solar cells 905, which are grouped intoserially connected strings 910. The strings 910 are connected togetherin series. For each string 910, a bypass diode 920 is provided so thatin the event of drop in power output of one string, that string may bebypassed via the respective diode 920. Additionally, one switchingdevice, such as FET or IGBT (insulated gate bipolar transistor), 925 isconnected in a by-pass configuration so as to bypass the respectivediode. Once it is sensed that current is flowing via one diode 920 (oronce the voltage across string 910 is sensed to be negative), itsrespective switching device 925 is activated. This directs the currentthrough the switching device, so that the loss of energy is drasticallyreduced. The sensing can be done by, for example, sensing the voltageacross the string or the current across the diode.

FIG. 10 illustrates another arrangement according to an embodiment ofthe invention for reducing the power loss in solar strings. In FIG. 10,the solar panel 1000 is made of solar cells 1005, which are grouped intoserially connected strings 1010. The strings 1010 are connected togetherin parallel. For each string 1010, a bypass switching device 1025, suchas FET or IGBT, is provided so that in the event of drop in power outputof one string, that string may be bypassed via the respective switchingdevice 1025. Once it is sensed that a string 1010 enters reverse bias(whether due to poor lighting or malfunction), the respective switchingdevice 1025 is turned on so that current is flowing via its respectiveswitching device 1025. The sensing can be done by, for example, sensingthe voltage or current of the string.

FIG. 11 illustrates an arrangement according to an embodiment of theinvention for bypassing a solar string. That is, FIG. 11 illustrates howa converter, such as, for example, the converter of FIG. 6, may beutilized to trigger the bypass of the solar string and/or a diodecoupled across a solar string. In FIG. 11, the solar panel 1100 is madeof solar cells 1105, which are grouped into serially connected strings1110. The strings 1110 are connected together in parallel. For eachstring 1110, a bypass diode 1120 is provided so that in the event ofdrop in power output of one string, that string may be bypassed via therespective diode 1120. However, as explained with respect to FIG. 10,the diodes may be eliminated. Additionally, one switching device, suchas FET or IGBT, 1125 is connected in a by-pass configuration so as tobypass the respective string 1110 and/or diode 1120. Once it is sensedthat a solar string enters reverse bias, its respective switching device1125 is activated by the controller 906. This directs the currentthrough the switching device 1125, so that the loss of energy isdrastically reduced. The sensing can be done by, for example, sensingthe voltage across the string or the current across the diode, asexplained with respect to elements 703 and 704 of FIG. 7.

The present invention has been described in relation to particularexamples, which are intended in all respects to be illustrative ratherthan restrictive. Those skilled in the art will appreciate that manydifferent combinations of hardware, software, and firmware will besuitable for practicing the present invention. Moreover, otherimplementations of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims and theirequivalents.

1. A method for harvesting power from a distributed power system havinga plurality of DC power sources and a plurality of DC power converters,the method comprising: coupling each of the power sources to arespective DC power converter; coupling the power converters in series,to thereby form a serial string; coupling the serial string to an inputof a DC/AC inverter; fixing voltage level at the input of said DC/ACinverter to a predetermined fixed constant value, thereby forcingcurrent flowing through the serial string to vary according to powerprovided by the power sources so as to maintain the fixed constantvalue.
 2. The method of claim 1, wherein said fixing is performed bysaid inverter.
 3. The method of claim 2, further comprising controllingpower output from each power source individually and individuallyvarying at least one of the input voltage and current to each respectiveconverter according to a predetermined criterion.
 4. The method of claim3, wherein said controlling power output includes tracking maximum powerpoint of the power sources, and said individually varying is performedby setting at least one of the input voltage and current so as to drawmaximum power from the power sources.
 5. The method of claim 1, furthercomprising: individually converting the input voltage and current ofeach converter to output power at current level dictated by the currentflowing through the serial string.
 6. The method of claim 1, furthercomprising limiting power output from each converter to a preset safelevel until a release signal has been detected.
 7. The method of claim1, further comprising monitoring for release signal before transferringpower to the inverter.
 8. The method of claim 1, further comprisingmonitoring load on each converter individually and limiting power outputfrom each converter to a preset safe level until the load reaches apreset value.
 9. The method of claim 1, further comprising monitoringpower output of at least one of the power source and DC power converterand directing current to a bypass when the power output exhibitspredetermined characteristics.
 10. The method of claim 1, furthercomprising individually operating each power converter to monitor andreport power related data, the power related data comprising at leastone of: input current to the converter, input voltage to the converter,temperature of the power source, input power to the converter, andavailable illumination.
 11. In a distributed power system having a DCpower source coupled to a load, a method for improving the reliabilityof components within the load, comprising: coupling the DC power sourceto the load; maintaining by the load the input voltage to the load at afixed predetermined constant voltage, thereby holding the input voltageat a set value, the fixed predetermined voltage being a safe operatingvoltage for the components within the load; varying the current input tothe load according to the power drawn from the DC power source.
 12. Themethod of claim 11, wherein the load includes a DC/AC inverter, and thestep of maintaining the input includes maintaining the input voltage ofthe DC/AC inverter.
 13. The method of claim 12, wherein coupling the DCpower source includes coupling a solar panel to a DC/DC converter, andcoupling the DC/DC converter to the DC/AC inverter.
 14. The method ofclaim 13, further comprising operating said converter to boost thevoltage obtained from said solar panel as soon as the solar panel startsto output electrical energy.
 15. A method for harvesting power from aplurality of solar panels, comprising: coupling each of the plurality ofsolar panels to one of a plurality of DC/DC converters; coupling theplurality of DC/DC converters in series to form a string; coupling thestring to an input of a DC/AC inverter; operating the DC/AC inverter tomaintain a fixed constant voltage at the input; operating each converterindividually to draw maximum power from its respective solar panel andto convert the power to be output at the string by individually varyingthe input voltage or current to each respective converter according to apredetermined criterion.
 16. The method according to claim 15, whereinoperating each converter individually to draw maximum power comprisesperforming maximum power point tracking individually for each converter.17. The method according to claim 16, wherein operating each converterindividually to convert the power comprises individually selecting powerconversion such that the voltages of all of the panels adds to theconstant voltage at the input of the inverter.
 18. The method accordingto claim 15, wherein operating the inverter to maintain a fixed constantvoltage at the input comprises coupling a shunt regulator across theinput of the DC/AC inverter.
 19. The method according to claim 15,further comprising coupling at least one additional string the DC/ACinverter, each of the additional strings comprises a plurality of solarpanels each coupled to a respective DC/DC converter and each converterbeing operated to individually perform maximum power point tracking. 20.The method according to claim 16, wherein operating each converterindividually to convert the power comprises individually selecting powerconversion such that the current of all of the panels varies so as tomaintain the constant voltage at the input of the inverter.